A single plate or single tube image pickup device serves to generate a picture signal for every picture element in synchronism with well know read-out scanning.
This type of image pickup device has a light receiving plane provided with a color filter means opposite a plane of incidence of an optical image of a subject. For example, the light receiving plane is formed by light-receiving elements corresponding to respective picture elements. For example, the color filter means is composed of red, blue and green groups of fine filters respectively arranged corresponding to the picture elements, so that a color picture signal is generated in the form of a multiplex signal. Accordingly, this type of image pickup device serves to separate chrominance signals corresponding to respective colors and also serves to form a luminance signal and a color difference signal based on the chrominance signals thus separated.
The arrangement of fine filters for respective color phases in the color filter results in poor resolution and the occurrence of spurious colors. Further, similar effects may be produced as a result of the method of processing the read-out color picture signal.
As a counter measure to the aforementioned problem, an invention relating to the arrangement of the color filter means has been disclosed in Japanese Patent Unexamined Publication No. 62-140585 which results in improvement of resolution and the reduction of spurious colors. According to the disclosed invention, there is provided a charge-coupled solid-state image pickup device (CCD) including green striped filters provided corresponding to odd columns (or even columns) of picture elements arranged along the vertical charge-transference direction thereof and red and blue mosaic filters arranged alternately along the vertical charge-transference direction and the horizontal direction perpendicular thereto. This arrangement is called "green striped and red/blue perfectly-checkered filter arrangement". The occurrence of wavy patterns can be reduced by the color filter means without lowering the resolution in the vertical direction because the vertical sampling period of the respective color filter is equal to the vertical picture element period.
A theory of occurrence of spurious colors is now described in the case of using an image pickup device having a perfectly-checkered filter arrangement according to the Japanese Patent Unexamined Publication No. 62-140585.
FIG. 12 shows a perfectly-checkered filter arrangement. When a luminance signal Y is formed based on chrominance signals read from the image pickup device having such a filter arrangement, it is required that the upper limit of the frequency band of the luminance signal Y extends near a frequency half that of the sampling frequency f.sub.s (that is, f.sub.s /2). Accordingly, it is a general practice that the cut-off frequency of a low-pass filter for setting the frequency band of the luminance signal Y is established to be f.sub.s /2.
When, for example, an image of a perfectly green subject is picked up under the aforementioned condition, chrominance signals of green are detected from picture elements having green (G) filters but no chrominance is detected from other picture elements having red (R) and blue (B) filters as shown in FIG. 13. This is because the red and green filters do not transmit green light. In the drawing, the symbol X represents portions corresponding to the red and blue picture elements, in which no signal appears.
If a high-band luminance signal having the sampling frequency f.sub.s is used for the reproduction of an image, the red (R) and blue (B) portions become black so that a green striped image is reproduced even though the subject is green. When the cut-off frequency of the low-pass filter is established to be f.sub.s /2, the red (R) and blue (B) filter portions are changed to green to make it possible to reproduce the green subject.
As described above, the limitation of the frequency band of the luminance signal Y to a frequency half the sampling frequency f.sub.s is very effective in the case where such a green subject is reproduced. When the same analysis as described above is applied to the case where an image of a red subject is picked up, chrominance signals are not generated in picture elements corresponding to the green and blue filters. Accordingly, if a luminance signal having the sampling frequency f.sub.s is formed to reproduce an image, red spots corresponding to the red filters appear on the black screen which make it impossible to reproduce the red subject. Furthermore, a problem arises as follows. Red signals are generated at a quarter of the sampling frequency as shown in FIG. 14 though the frequency band of the luminance signal Y is limited to half that of the sampling frequency. Accordingly, the red signals pass through the f.sub.s /2 low-pass filter and it is impossible to limit the frequency band. As a result, the red subject is reproduced in the form of red spots.
Further, in the case where an image of a blue subject is picked up, blue signals are generated as shown in FIG. 15. Accordingly, in this case, the same problem as described above with reference to FIG. 14 arises. When an ordinary subject containing many colors is picked up under the condition that color phases cannot be correctly reproduced as described above, spurious colors are produced.
If the frequency band of the luminance signal Y is limited to a quarter (f.sub.s /4) of the sampling frequency in order to be matched with the red and blue spatial frequency for the purpose of solving the aforementioned problem, the lowering of resolution or color reproducibility becomes a major problem in the case where a polycolor image of an ordinary subject containing many colors is picked up. However, it is not a major problem in the case where a monocolor image of a subject not requiring high resolution and having one color, red, blue or green is picked up.
Under such circumstances, an object of the present invention is to provide a signal processing system for forming a luminance signal and a color difference signal based on a color picture signal generated from a single plate or single tube image pickup device using a perfectly checkered arrangement color filter means to thereby make it possible to improve reproducibility and reduce spurious colors.